We all have different experiences....Seoul excels for girl watching imho.

Spend some time in bars in the street behind Hamilton in Itaewon on a Fri/Sat night and it's hard to know where to look!
Throw in a walk around COEX, Dongdaemun, Hongdae, uni areas like Sinchon or Hyehwa or shopping centres like Lotte / Shinsegae.

Beem me back!